Multilayer thick film circuits have been used for many years to increase circuit functionality per unit of area. Moreover, recent advances in circuit technology have placed new demands on dielectric materials for this use. Heretofore, most of the dielectric materials used in multiple circuits have been conventional thick film dielectric compositions. These are comprised of finely divided particles of dielectric solids and inorganic binders dispersed in an inert organic medium. Such thick film materials are usually applied by screen printing, though they may be applied by other means as well.
Thick film materials of this type are very important and will continue to be so. However, when applying these thick film materials by screen printing, it is difficult to obtain better than 8 mil (203 .mu.m) line and space resolution. In fact, to obtain even this level of performance, it is essential that all the screen printing variables such as screen quality, squeegee hardness, print speed, dispersion properties, etc., be most carefully controlled and constantly monitored to obtain good product yields. Similar problems exist, of course, with the use of thick film conductor and resistor materials.
One approach to this problem is (1) to apply a layer of the dielectric material to a substrate by means of dispersion in a photosensitive medium, (2) to expose the layer imagewise to actinic radiation, (3) to solvent develop the pattern to remove unexposed portions of the layer, and (4) to fire the remaining exposed portions of the pattern to remove all remaining organic materials and to sinter the inorganic materials. This technique is revealed in several issued patents such as U.S. Pat. No. 3,443,944 to Wise, U.S. Pat. No. 3,615,457 to Seibert, U.S. Pat. No. 3,958,996 and U.S. Pat. No. 3,982,941 to Inskip, U.S. Pat. No. 3,877,950 to Felten and U.S. Pat. No. 3,914,128 to Scheiber et al. None of these systems utilizes an inorganic binder. However, U.S. Pat. No. 3,355,291 to Baird et al. and U.S. Pat. No. 3,573,908 to Minetti describe a method for applying glass to a semiconductor device by applying a photosensitive paste of the glass, exposing, solvent developing and firing the exposed areas.
Notwithstanding the effectiveness of the prior art processes for applying such electronic materials as a paste, it would be even better to apply such materials as a dimensionally stable film. There are several advantages to the use of film, including: (1) better surface uniformity, (2) better layer thickness uniformity, (3) a thicker layer can be applied, thus taking fewer steps to achieve thick fired dielectric layers, (4) greater processing uniformity, (5) longer storage life, (6) minimum sensitivity to dirt pickup, and (7) no significant change in viscosity as compared to the viscosity changes which take place in pastes due to drying on the printing screen. Consequently, there is a strongly unmet need for ceramic dispersions which can be applied by either conventional methods, such as screen printing where appropriate, or as a laminated film where more exciting properties are required.